The present invention relates to the assessment of picture quality for video image signals, and more particularly to a harmonic method and apparatus for measuring blockiness in video image signals.
In digital video transmission where bandwidth is important, such as transmission over satellite link, the video signal is subjected to varying degrees of compression to decrease the bandwidth required for each video channel. The compression standards typically used, such as JPEG, MPEG or proprietary variants thereof, are xe2x80x9clossyxe2x80x9dxe2x80x94to achieve higher compression they allow distortions to occur in an image represented by the video signal. The amount of distortion is a function of the complexity of the image and of the number of bits per second (bit rate) a compression encoder is allowed to use. Ideally the amount of compression is maximized while still providing a video image or picture to the customer that is free of disturbing artifacts.
Current devices to analyze picture quality, such as the PQA200 Picture Quality Analyzer manufactured by Tektronix, Inc. of Beaverton, Oreg., USA, are reference-based. A video signal transmitted through a video system is compared with the original video signal as a reference video signal in a measurement device. The reference video signal is either stored in the measurement device or is transmitted via some other non-distorted pathway to the measurement device. Use of a reference video signal is necessary for extremely accurate algorithms, such as the Sarnoff Corporation JNDmetrix(trademark) human visual model algorithm. However, this means that measurements are only made on video signals whose contents are either known in advance or are immediately available, such as double-checking an encoder at the source.
Other potential methods of measuring discrete cosine transform (DCT) based codec degradations involve directly examining the coarseness of the quantization scale in the compressed video stream, optionally combined with a measure of the complexity of the original image transmitted by some means outside the video channelxe2x80x94a form of compressed reference. This method is not as accurate, and in any case can only make measurements on compressed video, not video that has been already decompressed and potentially passed through other systems, including other additional codecs, prior to end-user delivery.
What is desired is a method and apparatus that allows measurement of blockiness artifacts, such as incurred when compressing video using discrete cosine transforms, in situations where there is no prior knowledge of the blockiness period and in the presence of image-content generated noise.
Accordingly the present invention provides a harmonic method and apparatus for measuring blockiness in a video input signal. A power spectrum is obtained for a video field of the video input signal as the Fast Fourier transform of the sum of the lines in the video line after edge filtering. The largest frequency peaks from the higher frequency end of the power spectrum are initially selected, and a common period for the peaks is determined representing a common frequency interval. The amplitudes of the peaks at the common frequency interval are corrected for the contributions made by peaks at one-half and double the common frequency interval. The corrected amplitudes of the peaks at the common frequency interval are averaged and normalized to produce a blockiness metric for the video input signal.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.